Periodic Reciprocating Motion of a Polymer Gel on an Aqueous Phase

Dec 23, 2013 - A self-oscillating gel induced by the Belousov–Zhabotinsky (BZ) reaction was investigated on an aqueous phase. When the Ru-catalyst i...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/Langmuir

Periodic Reciprocating Motion of a Polymer Gel on an Aqueous Phase Synchronized with the Belousov−Zhabotinsky Reaction Satoshi Nakata,*,† Miyu Yoshii,† Seiichi Suzuki,‡ and Ryo Yoshida‡ †

Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan Department of Materials Engineering, School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan

‡

S Supporting Information *

ABSTRACT: A self-oscillating gel induced by the Belousov− Zhabotinsky (BZ) reaction was investigated on an aqueous phase. When the Ru-catalyst in the gel was rapidly oxidized, the gel was accelerated in a direction opposite to the side of oxidation. The gel then returned to its original position while the Ru-catalyst in the gel was slowly reduced. To clarify the mechanism of this periodic reciprocation of the gel, the contact angle between a sessile bubble and the gel and the timevariation of the adhesive force of the gel on the aqueous phase were measured. The experimental results suggest that the periodic reciprocation of the gel is driven by the periodic change in the contact angle of the gel induced by the BZ reaction.

1. INTRODUCTION The development of autonomous actuators or self-propelled objects that can transport either a substance or themselves on a small scale and mimic sensation in living organisms has been studied in the fields of medicine and engineering.1−3 Such actuators or objects not only can control motion without the need for an external on/off switch but also can sense the physical and chemical environment, such as taxis in living organisms. To induce the characteristic features of motion, such as oscillation, bifurcation, and synchronization, we have introduced nonlinearity to self-propelled objects on water.4−6 A selfoscillating polymer gel composed of N-isopropylacrylamide (NIPAAm) and [Ru(bpy)2(4-vinyl-4′-methylbpy)]2+ (Ru(bpy)2(vmbpy)) catalyst has been reported as a novel autonomous actuator.7−13 This gel exhibits periodic alternation between swelling and deswelling in synchronization with oxidation and reduction of the Ru-catalyst in the Belousov− Zhabotinsky (BZ) reaction.14−16 Although many kinds of selfoscillating gels have been synthesized and several interesting systems (e.g., mass transfer and robots9,10) have been reported, most systems have been examined as conformational or shape changing systems of the polymers and the gels in the solution. In this study, the oscillatory motion of a gel synchronized with the BZ reaction, which mimics water-walking insects driven by the change in the contact angle,17−19 was investigated on an aqueous phase. The force and contact angle around the gel on the aqueous phase were measured to clarify the driving force of motion. To clarify the influence of the surface tension on self-motion, Triton X-100 was added to the aqueous phase as a surfactant. The mechanism of oscillatory motion of the selfoscillating gel on the aqueous phase is discussed in relation to © XXXX American Chemical Society

the change in the contact angle of the gel synchronized with the BZ reaction.

2. EXPERIMENTAL SECTION A NIPAAm gel was synthesized based on a procedure reported previously.10 Poly(NIPAAm-co-Ru(bpy)3-co-AMPS) gels were prepared as follows: For a monomer solution, NIPAAm (623.7 mg), Ru(bpy) 2 (vmbpy) (65.0 mg), N,N′-methylenebisacrylamide (MBAAm, 11.2 mg, cross-linker), and 2,2′-azobis (isobutyronitrile) (AIBN, 26.6 mg, initiator) were dissolved in methanol (2 mL). 2Acrylamido-2-methylpropanesulfonic acid (AMPS, 22.0 mg) was dissolved in pure water (1.6 mL) and DMSO (0.4 mL). These solutions were mixed together as a monomer solution, and O2 molecules in the monomer solution were removed by nitrogen purging. The monomer solution without O2 was then injected into the space between two glass plates gapped with a silicone sheet (thickness 1.5 mm).20 The injected solution was then polymerized at 333 K for 20 h. After polymerization, the gel was soaked in pure ethanol for 1 week and then soaked in a graded series (75, 50, 25, and 0 vol %) of ethanol/water mixtures every day to remove unreacted monomers and carefully hydrate the gel. For each trial, the prepared gel film was carefully cut small. A piece of a symmetric gel (3.0 mm × 3.0 mm × 1.5 mm) was floated on a BZ aqueous solution. The asymmetric gel composed of the self-oscillating gel (BZ gel) disks and polyacrylamide (PAAm) gel was prepared as follows. First, a PAAm gel in the shape of a snowman was prepared as a nonactive gel to which the BZ gel disks were attached. Acrylamide (AAm), MBAAm (10 mol %), and Irgacure 651 (1 mol %) acting as a photoinitiator were dissolved in a mixture of methanol and water (50/50 vol %). The pregel solution was injected into the space between two glass plates Received: September 26, 2013 Revised: December 19, 2013

A

dx.doi.org/10.1021/la403675z | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

separated by a silicon rubber sheet (thickness 300 μm). UV light was irradiated through a photomask having a negative image of a snowman. After gelation, the glass plates were removed and the PAAm gel was immersed in pure water for one day. Then the PAAm gel was inserted into the space between two glass plates separated by the silicon rubber sheet. The pregel solution of the BZ gel composed of N-isopropylacrylamide (NIPAAm) monomer, Ru(bpy) 2(vmbpy) (0.9 mol %), N,N′-methylenebisacrylamide (MBAAm) (5 mol %), Irgacure 651, and methanol was injected around the PAAm gel. UV light was irradiated through a photomask having a negative image of micropatterned round shapes with a diameter of 600 or 900 μm. After gelation, the glass plates were removed and the gel composite was immersed in pure water for one day. The synthesized gel was immersed in a BZ aqueous solution composed of 0.084 M NaBrO3, 0.0625 M malonic acid, and 0.894 M nitric acid.10−12 The experiments were carried out in a thermostat water bath at 291 ± 1 K.10,11 The gel was monitored with a digital video camera (SONY HDR-CX590, minimum time-resolution of 1/30 s), and the images were analyzed by an image-processing system (National Institutes of Health, ImageJ). The green level was extracted from the RGB image to evaluate the BZ reaction on the gel because the colors green and orange correspond to the oxidation and reduced states, respectively. The contact angle at the gel/liquid interface was measured by using an argon bubble in contact with the gel.21 The force applied to the gel on the aqueous phase was measured with a surface tensiometer (CBVP-A3, Kyowa Interface Science Co. Ltd., Saitama, Japan). The volume of the aqueous phase was 2 mL, and the diameter of the Petri dish was 34 mm. To observe the convection in the aqueous phase, visualization particles (DIAION, HP20S, Mitsubishi Chemical Co., Tokyo, Japan, particle size 100−200 μm) were dispersed on the aqueous phase.

the maximum green level, which corresponded to when the entire area of the gel was oxidized. The gel then moved back with a decrease in the green area, which corresponded to reduction. The forward motion was faster than the reverse motion. The migration length for the forward motion was almost equal to that for the reverse motion because the cumulative speed for the forward motion was almost equal to that for the reverse motion. The period of reciprocation was 13.3 ± 2.1 min for eight trials. When a smaller piece of the gel (1 mm × 1 mm × 0.5 mm) which indicated homogeneous oscillation was floated on the aqueous phase, reciprocating motion synchronized with the BZ reaction was not observed. It was difficult to control the position of the wave generation on the gel. However, the gel was floated almost on the center of the Petri dish at the initial state. Simultaneous Measurement of the Green Level and the Force for a Self-Oscillating Gel. Figure 2 shows the

3. RESULTS Reciprocating Motion of a Symmetric Gel. Figure 1 shows the periodic motion of a self-oscillating gel on an

Figure 2. (a) Experimental system to measure the force and simultaneous measurement of (b-1) the green level and (b-2) the force for a self-oscillating gel that was fixed on the surface of a BZ solution.

time variation of (b-1) the green level and (b-2) the force for a self-oscillating gel that was fixed on the surface of the BZ solution. When a chemical wave was generated at one edge of the gel, the gel started to move, as shown in Figure 1. In addition, a change in the force is generated around the gel. Therefore, we analyzed the edge of the gel. The force at the air/ gel/liquid interface oscillated periodically in synchronization with the BZ reaction. When the gel was oxidized from the reduced state, the force increased. The force then decreased from its maximum value during reduction of the gel. The rapid oxidation and slow reduction were reflected by the time variation of the force. The period of oscillation for the measurement of the force was 17.0 ± 2.4 min for four trials. The surface tension of the aqueous solution was ∼45 mN m−1 after the trial. Measurement of the Contact Angle between Gel/ Aqueous and Air/Aqueous Interfaces. To measure the contact angle between gel/aqueous and air/aqueous interfaces, an argon bubble was placed in contact with a self-oscillating gel (see Figure S3 of the Supporting Information). Four trials were performed for each experiment. The contact angle in the reduced state (48 ± 3°) was larger than that in the oxidation state (42 ± 3°). In experiments on the dispersion of visualization particles in the aqueous phase, convective flow, which was synchronized with the BZ reaction of the gel, was not observed (see Figure S4 of the Supporting Information). To decrease the surface tension around the gel, 0.1 mM Triton X-100, which has a surface tension of 40 mN m−1 on pure

Figure 1. (a) Photograph of an experimental system (top view), (b) snapshots of reciprocating motion of a gel (slanted view), and (c) the time variation of (c-1) the percentage of green area and (c-2) the speed of a self-oscillating gel on a BZ solution. The horizontal bar in (c-2) corresponds to the data in (b).

aqueous phase of the BZ reaction (Movie 1 in the Supporting Information). Reciprocating motion synchronized with the BZ reaction was observed periodically. When a chemical wave, which corresponded to the oxidative state, was generated from a side of the gel, the gel was accelerated in the direction opposite wave generation. The speed of the gel became zero at B

dx.doi.org/10.1021/la403675z | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

rapidly accelerated by the oxidation in the BZ reaction. In contrast, the lower acceleration of the reverse motion corresponds to the slow reduction of the gel. The reason why the contact angle at Ru2+ is larger than that at Ru3+ is related to the work by other researchers.21 They reported that the contact angle at a deswelling state is larger than that at a swelling state. The change in the contact angle depending on the degree of swelling or deswelling was discussed in relation to the nonpolar and polar components of the gel.22 As the gel is fixed on the aqueous phase in Figure 2, the period of oscillation in Figure 2 is longer than that in Figure 1 because the exchange of chemicals in the gel in Figure 2 is difficult in comparison with the free gel in Figure 1. The effective contact length of the square BZ gel for a unidirectional motion is 3 mm which corresponds to the length of one edge. In contrast, the effective contact length for the snowman is significantly shorter than that for the square gel because of the short contact length, circular shape, and homogeneous oscillation. Therefore, the speed for Figure 3 is lower than that for Figure 1. Figure 2 suggests that the adhesive force of the gel on the aqueous phase changes periodically in synchronization with the BZ reaction and is high and low at the oxidation and reduced states, respectively. The time lag between the maximum force and the maximum speed in Figure 2 is generated because of the measurement of the green level on an edge of the gel. The measured force, F, is lγAWcos θ, where l is the peripheral length of the gel, γAW the surface tension around the gel at the air/ water interface, and θ the contact angle. Table 1 shows the

water, was added to the aqueous phase. Oscillatory motion synchronized with the BZ reaction was observed under this condition. Reciprocating Motion of an Asymmetric Gel. To clarify the relationship between the direction of motion and the direction of the chemical wave, a hybrid gel, which consisted of four smaller BZ gels and a larger acryl amide gel, was floated on the BZ aqueous solution, as shown in Figure 3 (Movie 2 in the

Table 1. Maximum and Minimum Values of F + Fv, F, cos θ, l, V, and γAW Based on the Experimental Results and the Equation F + Fv = l γAW cos θa

maximum (Ru3+) minimum (Ru2+) max/min

Figure 3. (a) Photograph of an experimental system (slanted view), (b) snapshots (top view) of a hybrid gel in the shape of a snowman, and (c) time variation of (c-1) the gray scale at one of the BZ gels (see thick dotted circle at t = 11 min) and (c-2) the speed at a point (see snapshot at t = 11 min) on the gel. The hybrid gel was composed of four smaller BZ gels (BZ in (b)) and a larger poly acryl amide gel (PAAm in (b)). F and R in (c-2) denote forward and reverse motion, respectively. The concentration of nitric acid was 0.5 M in this experiment.

F + FV (mN)

F (mN)

cos θ

l (mm)

V (mm3)

γAW (mN m−1)

0.326

0.193

0.743

11.0

11.3

39.89

0.296

0.173

0.669

10.8

10.9

40.97

1.10

1.12

1.11

1.02

1.04

0.97

a

Maximum and minimum values correspond to the oxidation (Ru3+) and reduced states (Ru2+), respectively.

relationship among F + Fv, F, cos θ, l, V, and γAW at the oxidation and reduced states. The measured F is expressed as F + Fv = F + ρVag = lγAW cos θ, where Fv is the buoyant force of the gel including a Pt wire, ρ (1.1 g mL−1) the density of the solution, Va (= V + VPt) the volume of the immersed gel (V) and Pt wire (VPt), and g the acceleration of gravity. Fv at the oxidation and reduced states are calculated as 0.133 mN and 0.123 mN, respectively. γAW obtained based on the equation is ∼40 mN m−1 which is similar to the surface tension of the aqueous phase (∼45 mN m−1) after the trial. Table 1 suggests that the change in l due to swelling and deswelling of the gel has little influence on the change in F + Fv and that the influence of γAW is very small. Therefore, the periodic changes in F and F + Fv are mainly due to those in θ rather than l and γAW. The lack of Marangoni flow synchronized with the BZ reaction suggests that the difference in γAW between the Ru2+ and Ru3+ states is very small. The periodic change in the interfacial tension at an air/water or oil/water interface was previously reported for ferroin systems on the BZ reaction.23,24

Supporting Information). That is, the system became asymmetric. When the oxidative reaction started from the reduced state, the hybrid gel rapidly accelerated in the direction of the acryl amide gel (forward motion). On the other hand, it moved slowly in the direction of the BZ gels when the reduction reaction progressed from the oxidation state (reverse motion). Thus, the hybrid gel showed periodic reciprocating motion synchronized with the BZ reaction.

4. DISCUSSION On the basis of the present experimental results and those in related works,13 we discuss the mechanism of self-propelled gel motion synchronized with the BZ reaction on an aqueous phase. If the oscillatory reaction occurs homogeneously around the symmetric gel, the gel cannot move because of the uniform surface tension around it. Figure 1 suggests that the gel is C

dx.doi.org/10.1021/la403675z | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Figure 4. Schematic illustration of the mechanism of one-dimensional reciprocating motion of a self-oscillating gel on a BZ solution.

■

Kitahata et al. reported that a BZ droplet on an oil phase moves in synchronization with the BZ reaction.25,26 The driving force of these systems is the change in the interfacial tension which is mainly due to the difference in the surface activity between ferroin (Fe2+) and ferriin (Fe3+) adsorbed on the interface. However, γAW has very little effect on the change in F because the catalyst exists on a meshwork in the gel rather than only at the interface. If the driving force is the change in γAW, reciprocating motion should not occur at 40 mN m−1, which is lower than the surface tension of the BZ aqueous phase.27 Therefore, the change in F is mainly due to the change in θ rather than to the change in l and γAW. On the basis of the experimental results, the above discussion, and related papers, we can suggest a mechanism for the reciprocating motion of the gel in synchronization with the BZ reaction. Because the whole gel is in a reduced state at state I, the gel is in a balanced state; therefore, it does not move. When the chemical wave occurs at one edge of the gel (left side in Figure 4), the contact angle at the left side decreases from θ0 to θ1 (state II). Because the lateral force (γAW(l/4) sin θ1) to the left is lower than that to the right, the gel accelerates to the right, i.e., to the side opposite the oxidation state. When the whole gel is in the oxidation state, the gel is in a balanced state at θ = θ1 (state III). The gel then slowly undergoes reduction from the left to the right and exhibits slow reverse motion because θ changes to θ0 and θ1, respectively, at the left and right sides (state IV). State IV returns to state I, and the cycle through states I, II, III, and IV is repeated. Figure 3 agrees with the suggested mechanism because the polymer gel without the Ru-catalyst (PAAm) is consistent with the direction of the motion.

ASSOCIATED CONTENT

S Supporting Information *

Movies 1 and 2 of reciprocating motion corresponding to Figures 1 and 3, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel/fax: +81-824-24-7409. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Dr. Y. Hara (AIST, Japan) for providing significant suggestions regarding the synthesis and properties of polymer gels and Mr. Yui Matsuda and Prof. Kentaro Ito for their technical assistance with the analysis of motion. This study was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (13234211) and the foundation of Sekisui Chemical Co., Ltd. to S. N.

■

REFERENCES

(1) Hong, Y.; Velegol, D.; Chaturvedic, N.; Sen, A. Biomimetic Behavior of Synthetic Particles: From Microscopic Randomness to Macroscopic Control. Phys. Chem. Chem. Phys. 2010, 12, 1423−1435. (2) Hamley, I. W. Nanotechnology with Soft Materials. Angew. Chem., Int. Ed. 2003, 42, 1692−1712. (3) Stano, P.; Luisi, P. L. Achievements and Open Questions in the Self-reproduction of Vesicles and Synthetic Minimal Cells. Chem. Commun. 2010, 46, 3639−3653. (4) Nagayama, M.; Yadome, M.; Murakami, M.; Kato, N.; Kirisaka, J.; Nakata, S. Bifurcation of Self-Motion Depending on the Reaction Order. Phys. Chem. Chem. Phys. 2009, 11, 1085−1090. (5) Iida, K.; Suematsu, N. J.; Miyahara, Y.; Kitahata, H.; Nagayama, M.; Nakata, S. Experimental and Theoretical Studies on the SelfMotion of a Phenanthroline Disk Coupled with Complex Formation. Phys. Chem. Chem. Phys. 2010, 12, 1557−1563. (6) Kohira, M. I.; Hayashima, Y.; Nagayama, M.; Nakata, S. Synchronized Self-Motion of Two Camphor Boats. Langmuir 2001, 17, 7124−7129. (7) Yoshida, R.; Takahashi, T.; Yamaguchi, T.; Ichijo, H. SelfOscillating Gel. J. Am. Chem. Soc. 1996, 118, 5134−5135. (8) Yoshida, R. Self-Oscillating Gels Driven by the BelousovZhabotinsky Reaction as Novel Smart Materials. Adv. Mater. 2010, 22, 3463−3483. (9) Shiraki, Y.; Yoshida, R. Autonomous Intestine-Like Motion of Tubular Self-Oscillating Gel. Angew. Chem., Int. Ed. 2012, 51, 6112− 6116. (10) Maeda, S.; Hara, Y.; Sakai, T.; Yoshida, R.; Hashimoto, S. SelfWalking Gel. Adv. Mater. 2007, 19, 3480−3484.

5. CONCLUSIONS The major finding of this study is that a gel with Ru-catalyst exhibits reciprocating motion on the BZ solution in synchronization with the BZ reaction. The mechanism of this reciprocating motion was discussed in relation to the adhesive force and the contact angle around the gel in synchronization with the BZ reaction. We concluded that the main driving force of the reciprocating motion is the change in the contact angle rather than the contact length (l) or the surface tension (γAW). Because the observation that the contact angle is the driving force behind this self-propelled motion is similar to the mechanism of movement for water-walking insects,17−19 the present system may be a useful artificial motor for understanding the collective motion of insects. D

dx.doi.org/10.1021/la403675z | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

(11) Mitsunaga, R.; Okeyoshi, K.; Yoshida, R. Design of Comb-Type Self-Oscillating Gel. Chem. Commun. 2013, 49, 4935−4937. (12) Masuda, T.; Hidaka, M.; Murase, Y.; Akimoto, A. M.; Nagase, K.; Okano, T.; Yoshida, R. Self-Oscillating Polymer Brushes. Angew. Chem., Int. Ed. 2013, 52, 7468−7471. (13) Ueki, T.; Yoshida, R. Self-Oscillating Micelles. Chem. Commun. 2013, 49, 6947−6949. (14) Zaikin, A. N.; Zhabotinsky, A. M. Concentration Wave Propagation in Two-dimensional Liquid-phase Self-oscillating System. Nature 1970, 225, 535−537. (15) Oscillations and Traveling Waves in Chemical Systems; Field, R. J., Burger, M., Eds.; Wiley: New York, 1985. (16) Chemical Waves and Patterns; Kapral, R., Showalter, K., Eds.; Kluwer Academic: Dordrecht, 1995. (17) Bush, J. W. M.; Hu, D. L. Walking on Water: Biolocomotion at the Interface. Annu. Rev. Fluid Mech. 2006, 38, 339−369. (18) Hu, D. L.; Bush, J. W. M. Meniscus-Climbing Insects. Nature 2005, 437, 733−736. (19) Prakash, M.; Quéré, D.; Bush, J. W. M. Surface Tension Transport of Prey by Feeding Shorebirds: The Capillary Ratchet. Science 2008, 320, 931−934. (20) Shiota, T.; Ikura, Y. S.; Nakata, S. Oscillation of a Polymer Gel Entrained with a Periodic Force. J. Phys. Chem. B 2013, 117, 2215− 2220. (21) Nakamura, T.; Hattori, M.; Kawasaki, H.; Miyamoto, K.; Tokita, M.; Komai, T. Surface Tension of the Polymer Network of a Gel. Phys. Rev. E 1996, 54, 1663−1668. (22) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Interfacial Lifshitz−van der Waals and Polar Interactions in Macroscopic Systems. Chem. Rev. 1988, 88, 927−941. (23) Yoshikawa, K.; Kusumi, T.; Ukitsu, M.; Nakata, S. Generation of Periodic Force with Oscillating Chemical Reaction. Chem. Phys. Lett. 1993, 211, 211−213. (24) Inomoto, O.; Abe, K.; Amemiya, T.; Yamaguchi, T.; Shoichi, K. Bromomalonic-Acid-Induced Transition from Trigger Wave to Big Wave in the Belousov-Zhabotinsky Reaction. Phys. Rev. E 2000, 61, 5326−5329. (25) Kitahata, H.; Aihara, R.; Magome, N.; Yoshikawa, K. Convective and Periodic Motion Driven by a Chemical Wave. J. Chem. Phys. 2002, 116, 5666−5672. (26) Kitahata, H.; Yoshinaga, N.; Nagai, K.; Sumino, Y. Spontaneous Motion of a Droplet Coupled with a Chemical Wave. Phys Rev. E 2011, 84, 015101. (27) Nakata, S.; Kirisaka, J. Characteristic Motion of a Camphanic Acid Disk on Water Depending on the Concentration of Triton X-100. J. Phys. Chem. B 2006, 110, 1856−1859.

E

dx.doi.org/10.1021/la403675z | Langmuir XXXX, XXX, XXX−XXX